In recent years, it has become necessary that image display devices have high-resolution and high picture quality, and it is desirable for such image display devices to have low power consumption and be thin, lightweight, and visible from wide angles. With such requirements, display devices (displays) have been developed where thin-film active elements (thin-film transistors, also referred to as TFTs) are formed on a glass substrate, with display elements then being formed on top.
In general, a substrate forming active elements is such that patterning and interconnects formed using metal are provided after forming a semiconductor film of silicon, e.g. amorphous silicon or polysilicon. Due to differences in the electrical characteristics of the active elements, the former requires Integrated Circuits (ICs) for drive use, and the latter is capable of forming circuits for drive use on the substrate. In liquid crystal displays (LCDs) currently widely used, the amorphous silicon type is widespread for larger screens, while the polysilicon type is more common in medium and small screens.
Typically, electroluminescent elements, for example organic light-emitting diodes (OLEDs), are used in combination with TFTs and utilize a voltage/current control operation so that current is controlled. The current/voltage control operation refers to the operation of applying a signal voltage to a TFT gate terminal so as to control current between two electrodes, one of which is connected to the OLED. As a result, it is possible to adjust the intensity of light emitted from the organic EL element and to control the display to the desired gradation.
However, in this configuration, the intensity of light emitted by the organic EL element is extremely sensitive to the TFT characteristics. In particular, for amorphous silicon TFTs (referred to as a-Si), it is known that comparatively large differences in electrical characteristics occur with time between neighboring pixels due to changes in transistor threshold voltage. This is a major cause of deterioration of the display quality of organic EL displays, in particular, screen uniformity. Uncompensated, this effect can lead to “burned-in” images on the screen. Additionally, changes in the EL element itself, such as forward voltage rise and efficiency loss, can cause image bum-in.
Goh et al. (IEEE Electron Device Letters, Vol. 24, No. 9, pp. 583-585) have proposed a pixel circuit with a precharge cycle before data loading to compensate for this effect. Compared to the standard OLED pixel circuit with a capacitor, a select transistor, a power transistor, and power, data, and select lines, Goh's circuit uses an additional control line and two additional switching transistors. Jung et al. (IMID '05 Digest, pp. 793-796) have proposed a similar circuit with an additional control line, an additional capacitor, and three additional transistors. While such circuits can be used to compensate for changes in the threshold voltage of the driving transistor, they add to the complexity of the display, thereby increasing the cost and the likelihood of defects in the manufactured product. Further, such circuitry generally comprises thin-film transistors (TFTs) and necessarily uses up a portion of the substrate area of the display. For bottom-emitting devices, where the aperture ratio is important, such additional circuitry reduces the aperture ratio, and can even make such bottom-emitting displays unusable. Thus, there exists a need to compensate for changes in the OLED emitter and in the electrical characteristics of the pixel circuitry in an OLED display without reducing the aperture ratio of such a display.